Pathological Characterization of African Swine Fever Viruses With Genetic Deletions Detected in South Korea

Abstract

African swine fever virus (ASFV) genotype II has been circulating in South Korea, causing substantial economic losses to the Korean pig industry since 2019. Genetic epidemiological investigations using whole-genome sequencing have been conducted to track the genetic evolution of ASFV. Two ASFV strains were detected in domestic pig farms in South Korea, one with a large deletion in the MGF 360-6L gene and the other in the MGF 360-21R gene. Phylogenetic analysis indicated that all Korean isolates belonged to the Asian subgroup of ASFV genotype II and were further divided into distinct subclusters of Korean African swine fever (ASF) group I. To identify the pathological changes caused by the deletion of MGF 360-6L and MGF 360-21R genes, we evaluated their pathogenicity in experimentally infected domestic pigs. No significant changes in pathogenicity were observed compared to other viruses evaluated in our previous studies. All inoculated pigs died 7-10 days post-inoculation (dpi), showing acute forms of illness with common pathological lesions. These results highlight that large genetic deletions can occur naturally in ASFV, but the deletions in MGF 360-6L and MGF 360-21R genes did not alter pathogenicity in domestic pigs. Further research is needed to understand the roles of these genes, especially in viral replication and pathogenicity in wild boars and ticks.

Keywords: African swine fever; South Korea; animal experiment; genetic characterization; next-generation sequencing; pig farms; virulence.

Figure 1

Diagram illustrating large gene deletion or replacement in the two ASFV isolates. (A) Deletion were identified within the 1942 bp segment with the nucleotides from end of MGF 360-4L gene to end of MGF 360-6L genes in the Pocheon strain. (B) Nucleotide position range where breakpoint of deletion is confirmed by cvPCR and elongation of complementary sequence ends in the Cheorwon strain.

Figure 2

Phylogenetic trees of genotype II African swine fever viruses. Taxa with red font are the ASFV isolates from domestic pigs and green font are the wild boars in South Korea. Two 2023 isolates analyzed in this study are marked with star. (A) Maximum likelihood phylogenetic tree. Phylogenetic trees were constructed using the maximum likelihood method in RAxML with 500 bootstrap replicates. Bootstrap values are shown in each node. (B) Time-scaled maximum clade credibility tree. The size and the color of the node (circle) indicate the posterior, which means the probability associated with forming one clade. Korean ASF was divided into three groups according to the posterior probability.

Figure 3

Comparative analysis of experimental pig groups in the two Korean isolates infection (red line; Pocheon2 strain (n = 5) and blue line; Cheorwon2 strain (n = 5)) and negative control groups (black line; n = 3). Results of (A) pig survival rates and the dynamics of (B) rectal temperature and (C) clinical score were denoted. Viremia and virus shedding condition were determined with viral copy number per 1 µL in (D) blood and experimental pig swab samples, including (E) oral, (F) nasal, and (G) rectal swabs. Also, (H) viral load compared to the swab samples versus blood. ⁣^∗p  < 0.05, ⁣^∗∗p  < 0.01.

Figure 4

Histological examinations in pigs experimentally infected with two Korean isolates. (A–C) Spleen, scale bar = 20 µm; (A) normal white pulp and (B, C) severe lymphoid necrosis on asterisks. (D–F) Gastrohepatic lymph nodes, scale bar = 50 µm; (D) normal lymphoid follicle and (E, F) severe lymphoid necrosis with hemorrhages on asterisks. (G–I) Liver, scale bar = 50 µm; (G) normal hepatocytes and sinusoid and (H, I) focal necrosis of hepatocytes with hemorrhages on asterisks. (J–L) Lungs, scale bar = 50 µm; (J) normal alveolar spaces and walls and interstitial pneumoniae with edema (arrows) in (K) and (L). (M–O) Kidneys, scale bar = 100 µm; (M) normal glomerulus and interstitium and (N, O) renal hemorrhages (arrows). (P–R) Spinal cord, scale bar = 50 µm; (P) normal blood vessel in thoracic spinal cord and (Q, R) vascular degeneration with weak perivascular lymphoid cell infiltrations (arrows).